There is no particular application system planned over any unlicensed spectrum, so the unlicensed spectrum resources can be shared by various wireless communication systems, e.g., Bluetooth, WiFi (Wireless Fidelity), etc., through preempting the resources. Sharing of the unlicensed spectrum resources between Long Term Evolution-Unlicensed (LTE-U) systems deployed by different operators, and between an LTE-U system and a WiFi or another wireless communication system has been studied as a focus and a difficulty. As specified by the 3GPP, the wireless communication systems shall coexist in a fair mode, where an unlicensed frequency band operates as a secondary carrier with the assistance of a primary carrier in a licensed frequency band. The Listen Before Talk (LBT) is a general contention access mode in the LTE-U system.
An 802.11 system operates with a channel access mechanism which is referred to as the Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) mechanism, and the WiFi system preempts a resource over an unlicensed frequency spectrum as illustrated in FIG. 1, where the particular process is as follows: firstly the WiFi system listens to a channel, and when the channel has been idle for a period of time reaching a Distributed channel access Inter-Frame Space (DIFS), it determines that the current channel is an idle channel; and then respective stations waiting for an access to the channel enter a random backoff stage, so that the stations can be avoided from colliding with each other over the same resource. For the sake of fairness, none of the stations can occupy any unlicensed spectrum resource for a long period of time, but each station shall release its occupied unlicensed spectrum resource after some period of time elapses, or an upper limit of the amount of transmitted data is reached, so that the resource can be preempted by another system.
In order to provide a flexible, fair, and adaptive channel access mechanism, the LBT technology is required in Europe to be applied in the unlicensed frequency bands of 5150 to 5350 MHz, and 5470 to 5725 MHz. Clear Channel Assessment (CCA) determines whether there is a signal being currently transmitted over the channel, as a result of energy detection to thereby determine whether the channel is occupied. Equipment in an unlicensed frequency band is categorized in the European Telecommunications Standards Institute (ETSI) standard into frame-based and load-based equipment corresponding respectively to two access mechanisms: Frame Based Equipment (FBE) and Load Based Equipment (LBE), where the LBE access mechanism is similar to the CSMA/CA mechanism of the WiFi system.
In the LBE channel access mechanism, a period of time for which, and a start point at which the channel is occupied by each transmission are varying, so extended CCA detection is performed before the channel is accessed. Where firstly a random factor N is generated according to a size q of a Contention Window (CW), and the channel will not be accessed until the channel has been idle for a period of time which is N times a CCA period of time (i.e., q) and the channel is idle, then data transmission procedure is initiated; and the channel can be occupied for a largest length of time, which is 13 milliseconds (ms). There are two options A and B for the LBE, where there is a fixed contention window of the option B, and this is a general LBE form; and FIG. 2 illustrates a schematic diagram of a channel access mechanism for the ESTI LBE option B.
As currently studied in the 3GPP, two LBT categories are defined as the LBT category 3 and the LBT category 4 respectively for transmission by the load-based equipment. There is a fixed contention window for the LBT category 3, and the ESTI LBE option B belongs to the LBT category 3; and the ESTI LBE option B is modified into the LBT category 4, and the load-based LBT is applied to the LBT category 4, and a contention window thereof is extended exponentially, or configured semi-statically. Since the contention window is extended exponentially in the WiFi system operating with the CSMA/CA access mechanism, in order to enable the LTE-U to coexist with the WiFi system in a fair mode, the LBT category 4 shall be applied at least to downlink transmission in the LTE-U system as required in the 3GPP.
In the LTE system, downlink control information is carried in a Physical Downlink Control Channel (PDCCH), and the number of resources for the PDCCH in carrier aggregation is relatively stable, and can be indicated semi-statically via higher-layer signaling, so a start position of a Physical Downlink Shared Channel (PDSCH) can be determined by decoding the PDCCH. The PDCCH is shared by multiple users, and terminals need to search for their control signaling throughout a control area under a certain rule. There are generally two categories of PDCCH resources to be detected blindly, where one category includes PDCCH resources in a common search space, and the other category includes PDCCH resources in a User Equipment (UE) specific search space; and there are different aggregation levels of a Control Channel Element (CCE), and also different numbers of corresponding channel resources, in each search space particularly as depicted in Table 1.
TABLE 1Search spaceThe number ofThe number ofTypeAggregation levelCCEsPDCCH resourcesUE specific16621264828162Common41648162
In the common search space, the aggregation level of CCEs is 4, and the number of PDCCH resources to be detected blindly is 4; or the aggregation level of CCEs is 8, and the number of PDCCH resources to be detected blindly is 2. For example, if there are only two Downlink Control Information (DCI) formats with different lengths to be detected blindly in the common search space, a terminal will detect blindly in the PDCCH common search space for a number 2*(4+2)=12 of times.
In the terminal specific search space, the aggregation level of CCEs can be 1, 2, 4, or 8, and the corresponding numbers of PDCCH resources to be detected blindly at the respective aggregation levels of CCEs are {6, 6, 2, 2} respectively. For example, if there are also only two DCI formats with different lengths to be detected blindly in the terminal specific search space, where one of the DCI formats is dependent upon a transmission mode in which the terminal currently operates (see Section 36.213 for details thereof), the terminal will detect blindly in the terminal specific common search space for a number 2*(6+6+2+2)=32 of times.
As can be apparent from the structure of an LTE frame, a signal is transmitted in the unit of a 1 ms sub-frame in the LTE system. However the LBT mechanism must be applied to downlink transmission in the LTE-U system, so it is very likely for a start point of time, at which an LTE-U signal is transmitted, to be any position in a channel access sub-frame instead of a start boundary of the sub-frame due to the factor of a contention access. Two sub-frame structures are currently supported in the standard, where the first sub-frame structure is referred to as a partial sub-frame, that is, a part of Orthogonal Frequency Division Multiplex (OFDM) symbols in the channel access sub-frame is a separate Transmission Time Interval (TTI) of less than 1 ms; and the other sub-frame structure is referred to as a floating sub-frame, that is, a part of OFDM symbols in a sub-frame, and a part of OFDM symbols in an adjacent sub-frame succeeding thereto constitute an integral TTI with the length of time, which is 1 ms. Accordingly there is a PDCCH/ePDCCH transmission mode over an unlicensed carrier apparently distinguished from a PDCCH/ePDCCH transmission mode in a licensed frequency band; no matter it is a partial sub-frame or a floating sub-frame.
There has been absent so far a solution to indicate a start position of a PDSCH by transmitting a PDCCH, without transmitting any initial signal, or any signaling indicating the start position of the PDSCH, in the case that an unlicensed frequency band is preempted with a random access.